The present invention relates to an electrically conducting fine carbon composite powder. More specifically, the present invention relates to fine carbon composite powder useful as an electrically conducting material for an electrode material used particularly in a Lithium(Li) battery, electrical double-layer capacitor and the like, and fine carbon composite powder useful for supporting a catalyst for use in a fuel battery, and also relates to the method for producing the powder, a catalyst for polymer electrolyte fuel battery using the carbon composite powder, a polymer electrolyte fuel battery cell and battery using the catalyst.
In recent years, use of carbon powder materials for Li battery, electrical double-layer capacitor, fuel battery and the like is increasing. Particularly, fine carbon powder represented by carbon black has heretofore been used as an electrical conductivity-imparting material (for example, added to a resin) or a sliding member and in addition thereto, is being widely used in a battery as an electrode material, an additive or a support for supporting a catalyst.
For example, in a Li battery, the fine carbon powder is used as an additive for maintaining the electrical conductivity between graphite powder particles which are the main material of the negative electrode. In a fuel cell, the fine carbon powder in the state of supporting platinum is coated on a carbon substrate and used as an electrode catalyst for the anode electrode, cathode electrode or the like. In an electrical double-layer capacitor, the fine carbon powder is used as an additive for maintaining the electrical conductivity between fine activated carbon particles which are the main material of the electrode. The carbon powder used in these applications is so-called submicron order sized carbon powder smaller than normal carbon powder having a size of xcexcm order obtained by the pulverization of coke or the like. By virtue of its small size, the carbon powder is useful as an electrical conducting material for improving the electrical conductivity between larger particles having a size of several xcexcm to tens of xcexcm.
This fine carbon power is required to have properties comparable to normal graphite powder, more specifically, good electrical conductivity as an electrode and in the case of a battery, electrical or chemical properties such that the carbonaceous member is resistant against a corrosion by an acid.
Carbon black is a material having properties satisfying these requirements to a certain extent and is used over a wide range. In general, carbon commonly obtained from coke is graphitized, for example, by heating at a high temperature with an attempt to stabilize chemically and improve the corrosion resistance. However, carbon black is a material difficult to graphitize and can be hardly graphitized by mere heating.
Therefore, for example, JP-A-62-246813 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a technique of adding boric acid to carbon black and heating the obtained slurry at a temperature of 1,000 to 2,000xc2x0 C. to reduce the d002 of carbon crystal, which is an index of showing the graphitization, even to 3.41 xc3x85 (0.341 nm), thereby attaining the graphitization. However, according to the study by the present inventors, d002 of carbon black cannot be lowered to less than 3.40 xc3x85 which is by far larger than the theoretical value for complete graphite (i.e. 3.354 A). Furthermore, mere heating for the graphitization fails in elevating the electrical conductivity as demanded.
Therefore the first object of the present invention is to obtain graphitized fine carbon powder having excellent crystallinity and thereby increased in the resistance against chemical corrosion and at the same time, improved in the electrical conductivity, and to provide a high performance catalyst for polymer electrolyte fuel battery and polymer electrolyte fuel battery using the catalyst.
In order to cope with recent environmental pollution issue due to exhaust gas from the internal combustion engine of an automobile or the like, an electric vehicle (EV) is being developed as an alternative in recent years. To keep up with this tendency, a fuel cell is increasingly expected to undertake the power source for EV and therefor a compact and high-performance fuel cell is demanded.
The fuel cell includes various types of fuel cells such as, according to the kind of electrolytic solution used, alkali type, phosphoric acid type, fused carbonate type and polymer electrolyte type. Among these, a polymer electrolyte fuel cell is attracting an attention as a power source for electric vehicle (EV) because of its operability at a lower temperature, easy handling and high output density.
For example, FIG. 2 shows a cross-sectional structure of one example of a unit cell used in a polymer electrolyte fuel battery. The fundamental structure of a unit cell is such that an ion exchange membrane 14 having appropriate water content is disposed in the center and sandwiched by the electrode comprising an anode catalyst layer 13 and a cathode catalyst layer 15. The anode catalyst layer 13 and the cathode catalyst layer 15 each is usually a sheet coated with a paste of carbon powder having supported thereon platinum or platinum alloy powder. The carbon powder is not particularly limited on the kind thereof as long as it has electrical conductivity, but those having a specific surface area large enough to support a catalyst are preferred and in general, carbon black is used.
In the outer side of the anode catalyst layer 13 and the cathode catalyst layer 15, electrically conducting anode gas-diffusing porous sheet 12 and cathode gas-diffusing porous sheet 16 for passing water and gas generated at the reaction are disposed respectively and in the outermost side, a carbon-based separator plate with grooves 11 is disposed to provide reaction gas passages, thereby constructing a unit cell. By stacking the many unit cells (several hundreds of cells) to form a multilayer structure, a high-output fuel battery is constructed.
Since the reaction of a fuel battery takes place on the catalyst layers, the greatest factor determining the energy amount of a fuel cell is how effectively to use the catalyst. In order to use the platinum catalyst with highest efficiency, the characteristics of carbon as the support such as electrical conductivity, adhesion of platinum (supporting property), corrosion resistance against electrolytic solution (ion) and heat conductivity need to be improved.
Furthermore, adhesion as a constituent element of a cell, for example, plane pressure to the ion exchange membrane and the gas diffusion sheet must be maintained over a long period of time.
The fuel battery having a structure such that hundreds of unit cells are stacked and the whole is cramped up under a predetermined cramping pressure is operated over a long period of time, the separator plate, the gas diffusion sheet and the like undergo creeping (a phenomenon that the thickness decreases) and although this creeping amount is small per unit cell, the sum total in the creeping amount of hundreds of cells as a whole is fairly large.
In this meaning, simple carbon black currently used as a support is not only deficient in the electrical conductivity necessary for a high-performance battery but also, when the battery is operated for a long period of time and the plane pressure between respective parts decreases to cause increase in the contact resistance between respective parts, the internal resistance of the battery increases and the battery performance disadvantageously deteriorates. Specifically, in the durability test over a time period in excess of ten hundreds of hours, the output often lowers to the level of 70 to 80%.
Therefore the second object of the present invention is to develop a catalyst support capable of compensating for the deterioration in the long-term durability of elemental carbon conventionally used as a catalyst support, to provide a catalyst support ensuring a higher maximum output, and a catalyst and a battery using the support.
As a result of extensive investigations by taking account of the above-described problems, the present inventors have found that by using carbon black that was considered to be hardly graphitized, submicron fine graphitized carbon powder having an X-ray plane spacing C0 value (double of d002) of less than 0.680 nm (namely, d002 is less than 3.40 xc3x85) can be obtained. Futhermore the present inventors succeeded to obtain a high-performance fuel battery by using the powder as a catalyst support for fuel battery.
In addition, the present inventors have found that by using an electrically conducting carbon composite powder for supporting catalyst, wherein carbon powder for supporting catalyst (carbon black) currently used is mixed with fibrous carbon, particularly with vapor grown carbon fiber, as a material for supporting a catalyst, a catalyst electrode having high output and high durability can be obtained.
Namely, the present invention relates to a carbon powder, the production method thereof, an electrically conducting carbon composite powder for supporting a catalyst having mixed therewith fibrous carbon, a catalyst for polymer electrolyte fuel battery, polymer electrolyte fuel battery cell, and polymer electrolyte fuel battery as described below.
1. Carbon powder having a primary particle size of 100 nm or less and an X-ray crystallite plane spacing C0 of less than 0.680 nm.
2. The carbon powder as described in 1 above, which has a primary particle size of 100 nm or less and an X-ray crystallite plane spacing C0 of 0.6730 nm or less.
3. The carbon powder as described in 1 or 2 above, which is carbon black.
4. The carbon powder as described in any one of 1 to 3 above, which shows a volume resistivity of 0.1 xcexa9xc2x7cm or less in the pressurized state under a pressure of 2 MPa.
5. The carbon powder as described in any one of 1 to 4 above, wherein boron content is in a range of 0.001 to 5% by mass.
6. The carbon powder as described in 5 above, wherein boron content is in a range of 0.1 to 5% by mass.
7. A method for producing the carbon powder as described in any one of 1 to 6 above, comprising adding boron carbide (B4C) to carbon black in an amount of 0.01 to 7% by mass in terms of boron and heat-treating the mixture at 2,500xc2x0 C. or more in a non-oxidative atmosphere.
8. The method for producing the carbon powder as described in 7 above, comprising adding boron carbide (B4C) to carbon black in an amount of 0.5 to 7% by mass in terms of boron.
9. The method for producing the carbon powder as described in 7 or 8 above, wherein the carbon black is at least one kind selected from the group consisting of oil furnace black, acetylene black, thermal black, and channel black.
10. An electrically conducting carbon composite powder for supporting a catalyst, comprising carbon powder as described in any one of 1 to 6 above, having mixed therewith fibrous carbon.
11. The electrically conducting carbon composite powder for supporting a catalyst as described in 10 above, wherein the fibrous carbon is vapor grown carbon fiber.
12. The electrically conducting carbon composite powder for supporting a catalyst as described in 11 above, wherein from 1 to 7% by mass of vapor grown carbon fiber is mixed with carbon powder.
13. The electrically conducting carbon composite powder for supporting a catalyst as described in any one of 10 to 12 above, wherein the carbon powder is heat-treated at a temperature of 2,500xc2x0 C. or more.
14. The electrically conducting carbon composite powder for supporting a catalyst as described in any one of 11 to 13 above, wherein the vapor grown carbon fiber is graphitized at a temperature of 2,500xc2x0 C. or more and boron content in the fiber is in a range of 0.001 to 5% by mass.
15. The electrically conducting carbon composite powder for supporting a catalyst as described in 14 above, wherein the boron content in the vapor grown carbon fiber is in a range of 0.1 to 5% by mass.
16. A catalyst for polymer electrolyte fuel battery, primarily comprising platinum or a platinum alloy and the carbon powder as described in any one of 1 to 6 above for supporting the catalyst.
17. A catalyst for polymer electrolyte fuel battery, primarily comprising platinum or a platinum alloy and the carbon composite powder as described in any one of 10 to 15 above for supporting the catalyst.
18. A polymer electrolyte fuel battery cell using the catalyst as described in 16 or 17 above for anode catalyst layer and/or cathode catalyst layer.
19. A solid polymer electrode fuel battery comprising at least more than two of the stacked polymer electrolyte fuel battery cell as described in 18 above.
20. A polymer electrolyte fuel battery using the catalyst as described in 16 or 17 above for anode and/or cathode electrode.